I just bought Minecraft and suddenly my whole Minecraft folder inside is gone; nothing except saves. I tried to Force Update but it said Not Downloaded. I downgraded to Snapshot 12w50a/12w50b, and I still get the error Not Downloaded. I installed LWJGL, but it's still not working. I'm using OS X 10.8.3.

I had a similar problem once, although it wasn't exactly the same. It seemed to be that when Minecraft said: not downloaded, it means that the Minecraft.net servers were down. Once they came back up, I deleted the application and re-downloaded it, and it installed fine.